Kamui Kobayashi topped the timesheets after 90 minutes of 'action' in opening practice for the Belgian Grand Prix, but the order was distinctly out of kilter after persistent - and often heavy - rain caused a headache for the twelve teams.

Although several drivers completed sighting laps in the opening minutes of the session, just about everyone's best time came at the end, after the teams realised that there was no point waiting for the sun to emerge, and that there may be some merit in learning about the conditions just in case the forecast for the rest of the weekend - which is supposed to be warm and dry - proves to be wrong.

After a sunny Thursday, the rain had been present through the night, and intensified between the GP3 practice session and F1's first outing for five weekends, handing the teams the dilemma of getting a feel for the conditions while not risking their machinery ahead of what could prove to be a pivotal weekend for the championship. In the end, Kobayashi was rewarded for his persistence, producing a time half a second clear of anyone else after completing a session-high 20 laps for Sauber.

The Japanese driver moved into P1 with a handful of minutes remaining, deposing Williams' Pastor Maldonado, who had occupied top spot for some time after a brief break in the precipitation prompted greater participation after a quiet opening hour. Daniel Ricciardo moved into third spot for Toro Rosso, ahead of team-mate Jean-Eric Vergne, with Mark Webber the best of the title contenders in fifth spot.

The Australian's championship rivals were generally towards the bottom of the order, although team-mate Sebastian Vettel took ninth spot and he joined Webber in racking up a double figure tally of laps. Webber, however, was fully 1.8secs off the pace, with Vettel a further 1.6secs adrift, underlining the caution being taken in the conditions. Points leader Fernando Alonso was 22nd overall, not posting a representative time as he clocked 2min 40.749secs in comparison to Kobayashi's 2min 11.389secs, and finished ahead of only Lotus' Kimi Raikkonen - who was timed at 2min 46.580secs - and Ferrari team-mate Felipe Massa, who had not recorded a timed lap before what appeared to be an engine failure in the closing moments. McLaren duo Lewis Hamilton and Jenson Button were 15th and 16th respectively after sitting out most of the session.

McLaren was among those to bring the most prominent development parts to their cars this weekend, with unusual sidepod wings appearing on the MP4-27. Sadly, the conditions would have prevented the gathering of useful data, leaving the teams hoping for some respite in FP2, instead of leaving them with major workloads on Saturday morning.